In golf playing, a long driving distance needs much more swing speed and thus the head air resistance during swing should be decreased as much as possible. For this object, it has been proposed a driver provided with a club-head made in a general shape of flattered cannonball having an isosceles triangle in the flat (the patent document 1). However, the thickness limitation of the flattered club-head could not make the air resistance smaller more than expected.
Generally vortex generated at the rear of the head causes the air resistance larger than any others, so that It has been also proposed to form a plurality of dimples on the crown part behind the burble point positioned at the middle of the head in order to decrease air turbulence behind the burble point.(the patent document 2).